The invention relates to a piezoelectric actuator, especially for actuating control valves or injection valves in motor vehicles, having an actuator body in the form of a multilayer laminate of layered plies of piezoelectric material and intervening metal or electrically conductive layers, acting as electrodes, which are contacted in alternation by metal outer electrodes, facing one another on the jacket side in the longitudinal direction of the actuator body, that are at least in the form of laminar electrode strips, and the outer electrodes are in contact with electric terminal leads for connecting the piezoelectric actuator to an electrical voltage.
One such piezoelectric actuator is known for instance from German Patent Disclosure DE 196 50 900 A1 of Robert Bosch GmbH.
Multi-ply piezoelectric actuators of this kind, when they are subjected to a pulsating electrical voltage at their electrode layers, execute analogously pulsating strokes, changing the spacing between their two face ends. In DE 196 50 900 A1 cited above, the metal outer electrodes that transmit the electrical voltage to the electrode layers and are located on both sides on the jacket side in the longitudinal direction of the actuator body, cover the active region of the actuator body.
During operation of the multi-ply piezoelectric actuators, because of the low tensile strength between the thin, stacked films of piezoceramic (such as lead zirconate titanate) and the metal or electrically conductive electrode layers, delaminations can occur that can spread outward in the form of cracks into the outer electrodes and cause current interruptions.
It is accordingly the object of the invention to make possible a multi-ply piezoelectric actuator of this generic type in such a way that current-interrupting cracks in the outer electrodes can be spanned, thus improving the security is of the electrical contacting and lengthening the service life of the piezoelectric actuator.
The nucleus of the invention resides in connecting the laminar electrode strips resting directly on the actuator body additionally with these spanning additional electrodes, which are in contact at multiple points, via narrow elastic feet, with the laminar electrode strips.
Preferably, these additional electrodes take the form of a double comb, with a comb back parallel to the electrode strips and two parallel rows of teeth laterally adjoining the comb back, the teeth of which form the elastic feet and are in contact with the laminar electrode strips.
One advantage of this kind of double comblike additional electrode is its simple, secure fastening to the electrode strip as a consequence of the thin, elastic teeth of the comb. In Particular, an arbitrarily thick solder layer can be used for contacting and additionally serves to dissipate heat from the actuator body.
The many design options of the double comb enable optimal fastening, simplification of production, and optimal checking of the fastening of the double comblike additional electrode to the laminar electrode strips.
The double comb acting as an additional electrode, after being attached to the laminar electrode strips, develops intrinsic stability on the actuator body that can be still further enhanced by an elastomer additive. The two rows of combs, over the service life of the piezoelectric actuator, offer a high degree of security from cracks that might occur.
Because the comb back is virtually closed and there is a heat-diverting elastomer, good cooling of the actuator body is possible.
A piezoelectric actuator according to the invention can advantageously be used for diesel or gasoline injection systems in the motor vehicle. A piezoelectric actuator of this kind, because it is capable of faster switching, can advantageously replace the electromagnetic system typically used as an adjusting device in injection systems.
The above characteristics and further advantageous characteristics of a piezoelectric actuator according to the invention will become even clearer from the ensuing description of a plurality of exemplary embodiments thereof, when the description is read in conjunction with the drawing.